The present invention relates to the field of signal processing; more particularly, the present invention relates to multistage step down filtering in radio-frequency (RF) front-ends in systems such as communications systems.
A communication system transfers information between a source and a destination. Generally, a communication system includes a transmitter and a receiver which transmit and receive information signals over some media, respectively. This media may be cable wiring or atmosphere. When communications occur over atmosphere, or air waves, they are commonly referred to as xe2x80x9cwirelessxe2x80x9d communications. Examples of wireless communications systems include digital cellular, packet data paging and digital cordless telephones, wireless modems, wireless local and wide area networks, digital satellite communications and personal communications networks.
Communication receivers have a number of different architectures, including superheterodyne and direct conversion. These approaches have a number of different problems associated with them. For example, in superheterodyne approaches, the low intermediate frequency (IF) required for use with certain filters results in an image channel that is fairly dose to the desired RF signal. To compensate for this close image channel, the receiver requires a sharp image-rejection filter in the front-end in order to provide enough image suppression prior to mixing. Direct conversion architectures can be designed so that the image signal and the desired channel are essentially the same so that there is no need for an image rejection filter. However, imperfect isolation between the local oscillator (LO) and the antenna results in the LO signal being detected by the antenna, thereby producing a DC component at the output of the receivers. Also, most of the signal amplification is usually performed at the baseband and consequently the signal level at the input of channel-selection filters may be low enough to be completely masked by the 1/f noise.
An approach that overcomes the problems described above for paging applications is shown in an article by Saneilevici et al., entitled xe2x80x9cA 900-MHz Transceiver Chipset for Two-way Paging Applications,xe2x80x9d IEEE Journal of Solid-state Circuits, Vol. 33, No. 12, December 1998. FIG. 7 is a block diagram of an RF front-end using a double down conversion approach. FIG. 8 is a schematic of the RF front-end shown in FIG. 7. The architecture uses an RF filter to reduce the LO interference and an LO reject filter to filter the LO from the desired signal. As the design is for paging applications, one problem with this approach is the difficulty in scaling it to support signals at higher frequencies. One reason that the approach is difficult to scale to higher signal bandwidths is because at higher bandwidths, RF filters and LO rejection filters produce higher thermal noise.
A radio-frequency (RF) front-end is described. In one embodiment, the RF front-end comprises a low noise amplifier (LNA), a first mixer, and an I/Q quadrature mixer. The LNA amplifies a received signal at a carrier frequency, and has inductive loads. The first mixer is coupled to the low noise amplifier and mixes an amplified received signal with a first local oscillator (LO) signal to if down convert the amplified received signal to an intermediate frequency (IF). The first mixer has inductive loads. The first frequency is related to the carrier frequency such that an image channel associated with the carrier and the frequency of the LO signal is outside the bandwidth of the inductive loads of the LNA. The I/Q quadrature mixer stage has a second mixer and a third mixer coupled to the first mixer. The I/Q quadrature mixer converts the amplified received signal at the IF to I and Q signals using a second LO signal related to the first LO signal.